Working vehicles such as a hydraulic shovel mainly comprise: an undercarriage freely movable in a particular direction; an upper rotating body placed on the undercarriage so as to be rotatable; and a work machine (i.e., front attachment) attached to the upper rotating body so as to be freely moved upward or downward. At the front of the upper rotating body is an operator's cab. Disposed in the operator's cab are, for example, an operating seat, an operation lever for various operations performed by the operator, and a monitor screen for displaying information about the quantity of remaining fuel. Further, disposed at the rear of the upper rotating body is an engine hood covering an engine, etc. At the rear end of the upper rotating body is a counterweight that balances the weight of the working vehicle.
In such a working vehicle, since an operator sitting in an operator's cab has a wide field of view in front of the operator's cab, the operator can easily notice obstacles or the like existing in front of the working vehicle. However, as for areas on both sides of the operator's cab and the area behind it, the engine hood or counter weight obstructs the operator's view, making it difficult for the operator to visually check if there are any obstacles or the like.
In view of this drawback, a conventional working vehicle is equipped with a camera in the rear part of the upper rotating body and images captured by the camera are displayed on a monitor screen installed in the operator's cab. Thus, for example, when moving the working vehicle backward or rotating the upper rotating body, an operator can check the area of a dead angle by using a monitor screen, thus ensuring the safety of work.
Japanese Patent Application Laid-Open No. 2002-201676 (Patent Document 1), as an example of an invention that relates to a working vehicle equipped with such a camera, discloses a warning device for a working vehicle. The warning device described in Patent Document 1 is characterized by accommodating a camera and other devices (alarm, etc.) in a recess or groove of particular dimensions formed in a counter weight. This prevents the camera, etc. accommodated in the upper rotating body from being damaged as a result of striking against an obstacle during rearward motion of the working vehicle or rotation of the upper rotating body.
When undertaking, for example, an excavation using the working vehicle described above, an operator operates the working vehicle while accessing a variety of information on a monitor screen, such as the quantity of fuel remaining and engine cooling water temperature, thereby confirming that the working vehicle is functioning properly. On the other hand, for example, when moving backward or rotating the upper rotating body, image information (images) displayed on the monitor screen is switched by the operator from image information corresponding to the aforesaid variety of information to image information corresponding to an image captured by the camera.
This makes it possible for an operator to check on the monitor screen the area of a dead angle as viewed from an operator's cab and, at the same time, undertake the work safely. In such a working vehicle equipped with a camera, the operator can perform various operations while suitably switching an image displayed on the monitor screen, in accordance with the nature of each work.
In relation to such a situation, Japanese Patent Application Laid-Open No. 2004-346643 (Patent Document 2) discloses a monitor device in which operability of switching between images on the monitor screen has been improved. In the monitor device described in Patent Document 2, an operation lever for an operator is provided with an image switching switch used for switching images displayed on the monitor screen. In addition, a mode selection switch is provided for selecting a mode (i.e., machine image display mode) for displaying, on the monitor screen, information about the state of the working vehicle or a mode (i.e., rear image display mode) for displaying on the monitor screen an image captured by a camera.
According to Patent Document 2, providing the aforesaid two kinds of switch, namely an image selection switch and a mode selection switch, makes it possible for an operator to easily switch the monitor screen and cope with complicated work. To be specific, as shown in FIG. 5, in excavating or the like using the working vehicle, an operator selects the machine image display mode with a mode selection switch, thereby undertaking a certain task while checking the state of the working vehicle on the monitor screen. In addition, by depressing an image selection switch on the operation lever as necessity requires, the display on the monitor screen can be switched to an image (i.e., image of the rear) captured by a camera. Further, each time the image switching switch is depressed, the display on the monitor screen is switched to the alternative.
In addition, the monitor device disclosed in Patent Document 2 is designed such that when a fixed time t1 elapses after the display on the monitor screen has been switched to a rear image display as a result of depressing the image switching switch during the machine image display mode, the display on the monitor screen automatically returns to a machine image display.
On the other hand, when the rear image display mode is selected using the mode selection switch, an image of the rear is displayed on the monitor screen, thus enabling an operator to check on the monitor screen the safety of the work. If, for example, the engine cooling water temperature abnormally rises or the quantity of fuel remaining drops to a critical level during the rear image display mode, an operator must be informed of the malfunction in the working vehicle. In the monitor device disclosed in Patent Document 2, if a malfunction has occurred in the working vehicle during the foregoing rear image display mode, the monitor screen automatically switches to a warning image display in order to inform the operator of the malfunction. Further, when a fixed time t2 elapses after switching to the warning image display has taken place, this monitor device automatically returns to the rear image display again.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-201676    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-346643